Cone-beam CT

ABSTRACT

Apparatus for CT cone beam scanning, comprising:
         a table for holding a subject;   a gantry;   a first detector array, having a given number of rows of detector elements spaced along an rotation axis of the gantry, mounted on the gantry;   a first plurality of x-ray sources mounted on the gantry for rotation about the patient table on a rotation axis, the number of rows being at least twice the number of sources; and   a controller that acquires data responsive to radiation from the sources from the detector array attenuated by at least part of the common volume of the subject irradiated by the plurality of radiation sources.

RELATED APPLICATIONS

This Application is a National Phase of PCT Patent Application No.PCT/IL2007/000462 having International filing date of Apr. 10, 2007, thecontents of which are incorporated herein by reference.

PCT Patent Application No. PCT/IL2007/000462 is also related to PCTPatent Application No. PCT/IL2007/000461 filed Apr. 10, 2007, thedisclosure of which is fully incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to computerized tomography (CT) systems. Someembodiments relate to multiple source cone-beam CT for rapid scanning.

BACKGROUND OF THE INVENTION

Many conventional CT scanner utilize X-ray radiation beams formed as oneor more narrow fans, where each fan, with its corresponding line ofdetectors, defines a slice in the subject. The scan is performed eitherslice-by-slice by moving the subject along the scanner rotation axis andstopping it while a radial scan is taken, or a helical scan is performedwhere the subject moves simultaneously with the scanning.

Multi-slice scanning as described above with currently availablescanners reaches a width in the order of 40 mm. Thus, to scan a typicalorgan requires translating the subject axially along the rotation axis.When moving organs are involved, however, especially the beating heart,the temporal resolution, i.e. ‘freezing’ the organ, is problematic.Various techniques were developed to tackle the problem such as gatingby ECG or learning the rhythm, with varying degrees of success.

One area of current interest is CT scanning of the heart, referred to as“cardiac CT.” This presents especially difficult problems since thelength of image that is required is of the order of 120 mm. Furthermore,for best results it is desirable to acquire all of the data in a onehalf plus fan beam angle rotation or in a single rotation. A half plusrotation can be made in 180 milliseconds, which, for proper phase withinthe diastole would result in sufficiently consistent data forreconstruction.

To overcome the limited width and reduce the numerous rotations ofradiation/detector around the subject, some approaches were suggestedwhere a multiplicity of radiation sources are used with linear ortwo-dimensional detectors. For example, U.S. Pat. Nos. 5,966,422,7,016,455, 7,039,152 and 7,039,153 describe CT systems utilizingmultiple fan beams sources arranged circumferentially about the CTrotation axis, each with a corresponding detector array. The disclosuresof these references are incorporated herein by reference.

U.S. Pat. No. 5,625,661, the disclosure of which is incorporated byreference, describes a system in which a plurality of axially displacedsources of X-rays are used to irradiate the patient. This system has anumber of rows of detectors equal to the number of sources. Since thenumber of sources would be limited and a required resolution wouldindicate a detector width of at most 2 mm, the system has a limitedpatient coverage and is described only in a spiral scan mode. Thus, thissystem does not allow for single or half rotation cardiac CT scanning.

U.S. Pat. Nos. 5,712,889 and 6,229,870, the disclosures of which areincorporated by reference, describe X ray sources and systems in whichplurality of axially displaced sources emit a plurality of parallel fanbeams, formed by array of septa, each irradiating different parts of thescanned subject and directed at different part of the detector array.These systems are inherently inefficient in utilization of the Xradiation since most radiation is absorbed by the septa while stillrequiring high power, generating high heat load, to energize thesources.

Cone-beam scanning is also known where a cone of radiation covers asubstantial section of the subject. In cone beam scanning an X-raysource irradiates the subject over a relatively larger solid angle andan opposing area detector on the other side of the subject detects thetransmitted radiation. Thus, a wide cone beam system covers a muchlarger portion of the subject relative to a fan beam or a narrow conebeam. With the larger area coverage, translating the subject is avoidedor more limited, and a fast scanning is possible with, in principle,better temporal resolution.

The simplest cone beam scan mode is a rotation of a single source (anddetector array) about the subject (circular source trajectory), althougha partial scan (less than 360°) is also possible.

The reconstruction algorithm used for reconstructing cone beam acquireddata is usually of the type called “Feldkamp” or “FDK” method (Feldkamp,L. A., L. Davis, and J. Kress (1984). Practical Cone-beam Algorithm.Journal of the Optical Society of America 1, 612-619, the disclosure ofwhich is incorporated herein by reference) or a derivative thereof. TheFDK algorithm is approximate and allows the scanned subject to extendoutside the projected region in the axial direction. Unfortunately, withthese types of solutions the reconstructed images have artifacts thatworsen with the distance from the mid plane (the source trajectoryplane) due to “data incompleteness”. Similar problems occur with otherreconstruction algorithms known in the art.

It is usually accepted that data completeness leading to exactreconstruction is available if the Tuy-Smith condition is met (Tuy, H(1983). An inversion formula for cone-beam reconstruction. SIAM Journalof Applied Mathematics 43, 546-552, the disclosure of which isincorporated herein by reference). This condition requires that thesource trajectory shall intersect every plane passing through thescanned subject volume of interest. This is definitely not the case fora cone beam single circular source trajectory.

FIG. 1 shows a simplified schematic axial cross-sectional presentationof the use of a single cone beam to irradiate a patient. The use of verylarge cone angles, sufficient to encompass the heart have been mootedand are under development. However, as can be understood from thefollowing discussion, they are expensive and unnecessarily irradiate thepatient.

FIG. 1 shows a single cone beam system 10 of the prior art having asingle source 12 and a detector array 14. It should be understood thateach detector element 16 shown in FIG. 1 corresponds to a linearcircumferential or planar linear array of detector elements. In atypical geometry each detector elements covers 0.5 mm width at an axisof rotation 58. Reference 18 represents a cylinder of reconstruction(patient). Cone beam 20 is seen to intersect patient 18 in threedistinct regions. A first region 22 is a region in which data can beacquired over all rotation angles and is referred to herein as theregion of full coverage. A second region 24 on each side of region 22corresponds to regions of data acquisition, in which data is availableover all rotation angles only at diameters smaller than the scannedsubject diameter. Region 26 on either side of region 24 corresponds tovolume irradiated only at particular gantry angles, in which data is notavailable for a half or a whole rotation. Since the region of fullcoverage is so much smaller than the spread of the beam at the detectorfor imaging the heart in a single rotation, the number of detector rowsmust be increased from typically 240 rows to cover the heart to about280 rows. Further, larger parts of the patient are subject to ionizingradiation than should optimally be irradiated. With the geometry of FIG.1, even in region 24 the data does not meet the Tuy-Smith condition,except for data close the focal spot rotation plan, resulting indegraded image quality for parts of the subject away from the rotationplan.

U.S. Pat. No. 6,996,204, the disclosure of which is incorporated hereinby reference describe a method in which a cone beam scanner withgeometry similar to that described in FIG. 1 is used in steps: cone beamattenuation data is acquired for one circular trajectory of the source,the scanned subject is axially translated relative to the source, conebeam data is acquired again for a second circular trajectory of thesource such that the two cone beams have a volume of overlap and imagesare reconstructed where for volume elements in the volume of overlapdata is used from both source trajectories. The method disclosed reducesthe amount of missing data and improves the resulted image quality.However, the method cannot be used for a single heart beat cardiacscanning since it involves two acquisition cycles separated in time.

U.S. Pat. Nos. 5,068,882 and 5,187,659, the disclosures of which areincorporated herein by reference describe systems wherein twooverlapping cone beam sources are provided and data is acquired fromboth within a single rotation of the sources relative to the scannedsubject. In the disclosed embodiments, the sources are displaced bothaxially and radially and each is provided with a two dimensionaldetector array, thus increasing cost and complexity of the systems.

U.S. Pat. Nos. 7,072,436 and 7,145,981 describe systems in which twodimensional arrays of radiation sources are provided with extent in bothaxial and angular directions. A common detector array is provided toreceive attenuated radiation from said sources.

The use of X-ray beams in medical imaging is well established. X Rayprojection imaging is used to obtain static or fluoroscopic images.Computerized tomography (CT) scanning uses a plurality of X-ray imagesto assemble a 3D reconstruction of an organ or a potion of a subject. Inboth cases, single focal spots are typically used. However, there areimaging systems using multiple focal spots.

U.S. Pat. No. 6,181,771 to Hell describes electro magnetic deflection ofan electron beam emanating from a cathode in an X-ray tube using a pairof electro-magnets placed on opposite sides of the beam. Hell isconcerned primarily with focusing the electron beam at a focal spot onan anode. The disclosure of this patent is fully incorporated herein byreference.

U.S. Pat. No. 6,483,890 to Malamud, describes an X-ray tube with asingle cathode and a single anode. The X-ray tubes described by Malamudincludes four deflection plates arranged in pairs. Malamud describesusing a changing voltage applied between each pair to cause an electronbeam to rotate in a circle about its nominal axial path through amidpoint between the four deflection plates. Malamud is concernedprimarily with focusing the beam at different places on the anode usinga predetermined temporal pattern. The disclosure of this patent is fullyincorporated herein by reference.

U.S. Pat. No. 4,689,809 to Sohval describes an X-ray tube with a similarphysical configuration to that of Malamud and/or Hell. Sohval, likeMalamud and Hell uses an electromagnetic field to deflect a beam ofelectrons originating from a cathode in an X-ray tube. The disclosure ofthis patent is fully incorporated herein by reference. U.S. Pat. No.4,637,040 describes a CT system utilizing this X-Ray tube.

U.S. Pat. No. 4,912,739 to Weiss also describes use of anelectromagnetic field to deflect a beam of electrons originating from acathode in an X-ray tube. The disclosure of this patent is fullyincorporated herein by reference.

U.S. Pat. No. 6,229,870 to Morgan describes a plurality of discreteanodes mounted within the vacuum envelope, the anodes selectivelygenerating a plurality of parallel x-ray beams. According to Morgan,each anode element is associated with a cathode assembly selectablyexcitable by a filament power supply. When selected, each cathodeassembly generates an electron stream which strikes the correspondinganode element and produces x-ray beams. The disclosure of this patent isfully incorporated herein by reference.

Grid control and Grid pulsing for x-ray tubes is well known and has beencommercialized, for example in the MRC line of X-Ray tubes of PhillipsMedical Systems.

SUMMARY OF THE INVENTION

An general aspect of some embodiments of the invention relate to anapparatus for CT cone beam scanning comprising a plurality of radiationcone beam sources spaced in a direction parallel to the rotation axis.

According to an aspect of some embodiments of the invention, theirradiation that passes through the patient utilizes at least part of acommon detector array. In some embodiments of the invention, data fromradiation generated by multiple beams is captured substantially over thesame area of a common detector array. In exemplary embodiments of theinvention only two sources are used. In exemplary embodiments of theinvention the number of detector rows is larger, optionally by a factorof two or more, than the number of sources. Optionally, the number ofrows of detectors is not a multiple of the number of sources and can beas high as 10 or 100 or 1000 times as large as the number of sources.

An aspect of some embodiments of the invention is concerned with theacquisition of a complete set of CT data sufficient for thereconstruction of the human heart in a 180 degree angle rotation (plusan additional angle, but substantially less than a full rotation) of thesources about the patient. In an embodiment of the invention, this datais generated substantially without irradiating the patient withsubstantial radiation not used in the reconstruction. Substantialradiation not used in the reconstruction is defined here as a situationwhere over 10% or 20% of the irradiated axis is not in the region offull coverage as defined above. Adding an additional angle for halfrotation acquisition in cone beam is well known. It is estimated that inthe present invention, an additional 20 to 40 degrees is added, althougha lesser amount could be added.

In some exemplary embodiments of the invention, the radiation sourcesare housed in a single vacuum housing. In some exemplary embodiments ofthe invention, the radiation sources are in separate vacuum volumes.

In some exemplary embodiments of the invention, each source is energizedand generates radiation in sequence. In some embodiments the frequencywith which the sources are energized is high enough so that data isacquired from both sources at substantially the same circumferentialposition. In some embodiments, the data is corrected to take intoaccount the small differences in rotation position of radiation from thetwo sources.

In some embodiments the switching is periodic at a frequency high enoughto provide sufficient number of views for each source and data fromsuccessive energizing is interpolated to be matched with data from theother source. A method for combining such data is taught in Th. Köhler,R. Proksa and M. Grass, “A fast and efficient method for sequentialcone-beam tomography”, Medical Physics November 2001 Volume 28, Issue11, pp. 2318-2327.

Rotation time in cardiac scanners would be in the range 200-400 msec perrotation. Lets assume it is 250 msec. A minimum of 500 views perrotation is needed but 750 views/rotation for each source would be abetter choice. The switching frequency to get 750 views×2 sources wouldbe 3 KHz (0.333 msec cycle, each source is 0.176 msec on and 0.167 msecoff).

In other embodiments the sources are energized closely spaced in timeand then are energized again after the source has rotated by a smallangle.

In exemplary embodiments of the invention, the plurality of sourcesradiate in cycles so that an alternation cycle of the plurality ofsources is short relative to the CT rotation or scanning time. Forexample, the cycle frequency is in the order of kilocycles per second(KHz).

Optionally, a plurality of sources are energized together but at leastone is electrically blocked (due to electrostatic or electromagneticforces). Optionally, at least one source is electro-mechanically,pneumatically, or mechanically blocked and unblocked.

In exemplary embodiments of the invention, the detector, or a partthereof, or a dimension thereof, is substantially perpendicular to thebeam center of at least one of the radiation sources.

In some embodiments of the invention, the cone beam is asymmetrical. Insome embodiments an edge of each cone is substantially perpendicular toa detector row, generally the row on the same axial end of the detectoras the source. In other embodiments the edge of the cone is notperpendicular, but extends outward from the center by 1-3 degrees.However, this small amount of deviation causes less than 10% ofirradiated length of center line which is not reconstructed, undernormal conditions.

In exemplary embodiments of the invention, the detector is substantiallyplanar. In exemplary embodiments of the invention, the detector iscurved. Optionally, the detector forms all or part of a circularcylinder or a sphere.

In exemplary embodiments of the invention, the plurality of radiationsources, and optionally the common detector, rotate around the subject,substantially about the CT rotation axis. In other exemplary embodimentsof the invention, the subject rotates about the rotation axis (forexample, in industrial applications).

In exemplary embodiments of the invention, attenuation data from themultiple sources are weighted for volume elements to provide data forreconstruction, as is well known in the art of cone beam reconstruction,for example, using the above referenced paper by Kohler, et al.Optionally, the weight is responsive to the path of the respectiveradiation beam to the detector and/or the volume. Optionally andadditionally, the weight is responsive to the angle and/or the length ofthe path.

In exemplary embodiments of the invention, reconstruction methods knownin the art are employed using data acquired from the detector due toattenuated radiation irradiated on a region by at least two radiationsources. Optionally and additionally, the acquired data is weightedresponsive to a radiation source.

An aspect of some embodiments of the invention relates to a vacuumenclosure containing a cathode assembly. The vacuum enclosure alsocontains at least two target areas for electron beams emitted from thecathode assembly. Two or more voltage gates are situated between thecathode assembly and the target areas. In an exemplary embodiment of theinvention, each gate comprises a grid assembly adapted to operate in atleast two modes. In a first mode, when electrified to a first voltage,the grid assembly provides a negative voltage relative to the cathode ofsufficient strength to block a projected electron beam to the associatedtarget. When electrified to a second voltage the grid assembly directs abeam at its respective target to generate X-rays from the selectedtarget.

In an embodiment of the invention, the targets are on separate anodes.Alternatively, they are on the same anode structure.

In an embodiment of the invention, a single cathode is employed.Alternatively, the cathode assembly comprises two adjacent cathodesfacing in directions, each adapted to emit a beam toward its associatedtarget.

An aspect of some embodiments of the invention relates to an anodestructure for the generation of X-Rays from two targets. According tothis aspect, the targets face each other. That is to say that they aresituated at positions on an anode or anodes such that the interior anglebetween of the normal to the surfaces of the anode or anodes on whichthey are situated is between 0° and less than 180°. It should beunderstood that an interior angle of 180° corresponds to the twosurfaces directly facing each other and that 0° correspond to thesurfaces being parallel. In various embodiments of the invention theinterior angle is between 45° and 135°, more preferably between 75° and105°. Where the normals do not intersect, the interior angle is definedas the interior angle of the projections of the two normals on a planesituated halfway between the two normals. In actual design of interestthe angle will be around 150-170 Deg, far from 75-105.

In an embodiment of the invention the x-ray beams formed on the twotargets which face each other generate overlapping X-Ray beams.

In some embodiments of the invention that incorporate both embodiments asingle cathode is used to irradiate both targets, which face each other.

In an exemplary embodiment of the invention, alternate direction of thebeam at the two targets alternately generates X-rays from the selectedtarget. In an exemplary embodiment of the invention, alternation occursat 1-5 kHz or lower or higher frequencies, depending on the applicationto which the tube is applied. In some embodiments e.g. when the tube isused in fluoroscopy systems switching frequency may be 25/30 or 50/60 Hzor similar frequencies.

An embodiment of the invention incorporates a modulation mechanism whichalternately applies negative high voltages to at least one of two ormore voltage gates. In an exemplary embodiment of the invention,application of the negative high voltage a gate serves to “close” thegate with respect to an incident electron beam. Conversely, a gate towhich a lower (optionally zero) voltage is applied is “open” withrespect to an incident electron beam. Optionally, the lower voltage isapplied to a gate to focus the electron beam to a desired degree withrespect to a selected anode. Optionally, the gate is comprised ofmultiple electrodes and differential electrification of the electrodesserves to shift the beam (and x-ray source) slightly, in the manner ofShoval or one of the other references which provide more than onesource, in a single vacuum envelope by steering the beam as referencedabove.

In an exemplary embodiment of the invention, one or more closed gatesprevent transmission of an electron beam to one or more anodes in anX-ray tube while one or more open gates permit transmission of anelectron beam to one or more anodes in the same X-ray tube.

In an exemplary embodiment of the invention, a single X-ray tubecomprising a cathode assembly, two or more targets on a one or moreanodes and voltage gates interposed between them generates overlappingcones of X-ray beams. Optionally, the overlap is volumetric and nottemporal.

Optionally, control circuitry regulates a duty cycle of the voltagegate. In an exemplary embodiment of the invention, the control circuitrysynchronizes duty cycles of two voltage gates to provide oscillationbetween the gates. Optionally, the control circuitry synchronizes dutycycles of three or more voltage gates to provide rotation, or any otherprogram, between the gates.

In an embodiment of the invention an X-Ray tube of the invention is usedas a source of X-Rays for a CT scanner, such as that described in theapplications incorporated by reference in the background applicationssection and in the present application.

In various embodiments of the invention the distance between the targetsvaries from about 5 cm to 10, 15 or more cm. An effective lower boundbetween the targets is provided by the need to have the cathode and gridstructures substantially between the targets. The actual distance isdependent on the use for the tube. For cardiac CT applications, such asthat described herein, 7-12 cm is a desirable distance range, since thisallows for the acquisition of projection data for the heart during asingle rotation of the tube around the patient. For other applicationsand for industrial applications, smaller distances may be appropriate.

In an exemplary embodiment of the invention, when used in some CTapplications described herein, the cone beam is asymmetrical, with thesmaller angle toward the heel of the anode. This allows for a smalleranode angle and an increased area of impingement on the anode for thesame effective spot size. This allows for increased peak power handling.In a typical CT application each anode angle could be 5-10 deg, filamentlength 4-10 mm, effective focal spot size viewed at rotation plan0.6-1.5 mm. Power rating depends on Anode diameter and rotation speed.In general, for CT applications a power rating of 80 kW is considered tobe suitable. However, for the dual spot tube, the rating per target maybe as low as 40 kW. In other applications the beams are symmetrical.

In an exemplary embodiment of the invention, spatial and/or temporaldistribution of an electron beam from a cathode among two or more anodescontributes to a reduction in heat load on the anodes.

There is thus provided, in accordance with an embodiment of theinvention, a method for CT cone beam scanning, comprising:

(a) relatively rotating a plurality of alternating cone beam radiationsources to an object, the sources being spaced parallel to the rotationaxis, and radiating through a common portion of the subject onto acommon detector array having axially spaced rows of detector elements,wherein the number of rows is greater than the number of plurality ofsources; and

(b) acquiring from at least a part of the detector, data responsive toradiation attenuated by at least part of the common volume of thesubject irradiated by the plurality of radiation sources by at least afactor of two.

Optionally, the method comprises

(c) reconstructing an image of the subject where the image of at leastpart of the common volume of the subject is reconstructed fromattenuation data originating from a plurality of radiation sources.

In an embodiment of the invention, the data is acquired in a singlerotation or less of said sources relative to subject.

There is further provided, in accordance with an embodiment of theinvention a method for CT cone beam scanning, comprising:

(a) relatively rotating a plurality of alternating cone beam radiationsources to an object, the sources being spaced parallel to the rotationaxis, and radiating through a common portion of the subject onto acommon detector array having axially spaced rows of detector elements;

(b) acquiring from at least a part of the detector, data responsive toradiation attenuated by at least part of the common volume of thesubject irradiated by the plurality of radiation sources; and

(c) reconstructing an image of the subject where the image of at leastpart of the common volume of the subject is reconstructed fromattenuation data originating from a plurality of radiation sources,

wherein said image is acquired in a single rotation or less of saidsources relative to subject.

In an embodiment of the invention, the image is the image of a beatinghuman heart.

In an embodiment of the invention, substantially all of the radiationilluminating the object is used in the reconstruction.

Optionally, the patient is stationary during said acquiring.

Optionally, the plurality of sources comprises two or three sources.

In an embodiment of the invention, acquiring data is limited to a periodof rotation of 220 degrees or less.

In an embodiment of the invention, all of the rows are illuminated byeach of the sources. Alternatively, fewer than all of the rows areilluminated by both sources. Preferably, at least one source illuminateseach row.

In an embodiment of the invention, cone beams from at least two sourcesare asymmetrical respective of the source trajectory plane.

In an embodiment of the invention, one edge of the asymmetrical conebeam is perpendicular to the detector array. Alternatively oradditionally, the cone beams from at least one source is symmetricalrespective of the source trajectory plane.

In an embodiment of the invention, the method includes alternatelyenergizing the radiation sources. Optionally, the plurality of sourcesradiate in cycles so that an alternation cycle of the plurality ofsources is short relative to the CT rotation or scanning time.

In an embodiment of the invention, data is acquired in synchronizationwith a signal, optionally at least one of an ECG signal or bloodpressure signal.

In an embodiment of the invention, x-ray sources revolve around theobject a plurality of times. Optionally, data is collected during saidplurality of rotations. Optionally, data from different revolutions isaveraged to reduce the noise in a reconstructed image. Optionally datafrom different revolutions is binned to form data from a given portionof a heart cycle and wherein said binned data is used to reconstruct aheart. Optionally, the data is used to reconstruct a plurality ofperiods within a heart cycle.

In an embodiment of the invention, rotationally moving comprises movingthe radiation sources around the subject.

There is further provided, in accordance with an embodiment of theinvention a method for CT cone beam scanning, comprising:

(a) relatively rotating a plurality of alternating cone beam radiationsources to an object, and radiating through at least one detector arrayhaving axially spaced rows of detector elements;

(b) acquiring from at least a part of the at least one the detector,data responsive to radiation attenuated by at least part of the commonvolume of the subject irradiated by the plurality of radiation sourcesby at least a factor of two; and

(c) reconstructing an image of the subject where the image of at leastpart of the common volume of the subject is reconstructed fromattenuation data originating from a plurality of radiation sources,utilizing substantially all of the radiation as defined herein.

There is further provided, in accordance with an embodiment of theinvention, apparatus for CT cone beam scanning, comprising.

a table for holding a subject;

a gantry;

a first detector array, having a given number of rows of detectorelements spaced along an rotation axis of the gantry, mounted on thegantry;

a first plurality of x-ray sources mounted on the gantry for rotationabout the patient table on a rotation axis, the number of rows being atleast twice the number of sources; and

a controller that acquires data responsive to radiation from the sourcesfrom the detector array attenuated by at least part of the common volumeof the subject irradiated by the plurality of radiation sources.

In an embodiment of the invention, the controller reconstructs an imageof the subject where the image of at least part of the common volume ofthe subject is reconstructed from attenuation data originating from aplurality of radiation sources. Optionally, the controller is adapted toreconstruct said data acquired in a single rotation or less of saidsources relative to subject.

There is further provided, in accordance with an embodiment of theinvention, apparatus for CT cone beam scanning, comprising:

a table for holding a subject;

a gantry;

a first detector array, having a given number of rows of detectorelements spaced along an rotation axis of the gantry, mounted on thegantry;

a first plurality of x-ray sources mounted on the gantry for rotationabout the patient table on a rotation axis; and

a controller that acquires data responsive to radiation from the sourcesfrom the detector array attenuated by at least part of the common volumeof the subject irradiated by the plurality of radiation sources andreconstructs an image of the subject where the image of at least part ofthe common volume of the subject is reconstructed from attenuation dataoriginating from a plurality of radiation sources and acquired from dataacquired over a single rotation or less of said sources relative tosubject.

In an embodiment of the invention, substantially all of the radiationilluminating the object is used in the reconstruction.

In an embodiment of the invention, the plurality of sources comprisestwo or three sources.

In an embodiment of the invention, acquiring data is limited to a periodof rotation of 220 degrees or less.

Optionally, all of the rows are illuminated by each of the sources.Alternatively, fewer than all of the rows are illuminated by bothsources. Optionally, at least one source illuminates each row.

In an embodiment of the invention, the cone beams from at least twosources are asymmetrical respective of the source trajectory plane.Optionally, one edge of the asymmetrical cone beam is perpendicular tothe detector array.

In an embodiment of the invention, the cone beams from at least onesource is symmetrical respective of the source trajectory plane.

In an embodiment of the invention, the controller includes a powersupply alternately energizing the radiation sources.

In an embodiment of the invention, the plurality of sources radiate incycles so that an alternation cycle of the plurality of sources is shortrelative to the CT rotation or scanning time.

In an embodiment of the invention, the apparatus includes:

a second detector array, having a given number of rows of detectorelements spaced along an rotation axis of the gantry, mounted on thegantry;

a second plurality of x-ray sources mounted on the gantry for rotationabout the patient table on a rotation axis, the number of rows being atleast twice the number of sources; and

a controller that acquires data responsive to radiation from the sourcesfrom both the first and second detector arrays attenuated by at leastpart of the common volume of the subject irradiated by the plurality ofradiation sources.

In an embodiment of the invention, the second detector array and thesecond plurality of x-ray sources are circumferentially offset from thefirst detector array and first plurality of x-ray sources. Optionally,the second detector array and the second plurality of x-ray sources arenot axially offset from the first detector array and first plurality ofx-ray sources.

In an embodiment of the invention, the first and second x-ray sourcescomprise sources axially spaced by a given distance and wherein thesecond plurality of x-ray sources are axially offset from the firstplurality of x-ray sources by an amount equal to one-half the givendistance.

In an embodiment of the invention, the first and second x-ray sourcescomprise sources axially spaced by a given distance and wherein thesecond plurality of x-ray sources are axially offset from the firstplurality of x-ray sources by an amount equal to the given distance.

In an embodiment of the invention, the first and second x-ray sourcescomprise sources axially spaced by a given distance and wherein thesecond plurality of x-ray sources are axially offset from the firstplurality of x-ray sources by an amount equal to twice the givendistance. Optionally, the plurality of sources comprise two symmetricalcone beam sources.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples of embodiments of the present invention aredescribed with reference to figures listed below. Equivalent structures,elements or parts that appear in some figures are labeled with the samenumerals. Dimensions of components and features shown in the figures arechosen for convenience or clarity of presentation and are notnecessarily shown to scale, except where indicated.

FIG. 1 shows a single cone beam system of the prior art having a singlesource and a detector array;

FIG. 2 is a view similar to that of FIG. 1, showing the use of twosources to reduce radiation to the patient while providing complete datain a half rotation, in accordance with an exemplary embodiment of theinvention;

FIGS. 3 and 4 show variations on the embodiment of FIG. 2;

FIG. 5 schematically illustrates a side view of a setup of a CT scannersimilar to that shown in FIG. 3, comprising three cone beam sources, inaccordance with exemplary embodiments of the invention;

FIG. 6 schematically illustrates a trans-axial view of a CT scannerhaving circumferentially offset groups of sources, in accordance withexemplary embodiments of the invention; and

FIG. 7 schematically illustrates a trans-axial view of an alternative CTscanner having circumferentially offset groups of sources, in accordancewith exemplary embodiments of the invention;

FIG. 8 schematically illustrates an anti-scatter grid with septa alignedparallel to the rotation axis, in accordance with an exemplaryembodiment of the invention.

FIG. 9A schematically depicts relevant portions of an exemplary X-raytube according to an embodiment of the invention;

FIG. 9B is a cross-sectional view of the cathode assembly of FIG. 9A;

FIG. 9C schematically depicts relevant portions of an exemplary X-raytube according to an embodiment of the invention;

FIG. 9D is a cross-sectional view of the cathode assembly of FIG. 9C;

FIG. 9E schematically depicts relevant portions of a third exemplaryX-ray tube according to an embodiment of the invention;

FIG. 10A is a schematic representation of control circuitry adapted foralternately producing an X-ray beam from at least two anodes in a singleX-ray tube according to an exemplary embodiment of the invention;

FIG. 10B is a schematic representation of control circuitry adapted foralternately producing an X-ray beam from at least two anodes in a singleX-ray tube according to an exemplary embodiment of the invention and inaddition deflecting the beam slightly so that the tube can emit beamsfrom four targets;

FIGS. 11A and 11B schematically show voltage wave forms for theenergizing X-ray tubes using the circuits of FIGS. 10A and 10Brespectively.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2 is a view similar to that of FIG. 1, showing a system 50utilizing two sources 52 and 54 to apply radiation to patient whileproviding attenuation data in a half rotation of the sources oroptionally a larger rotation angle, in accordance with an exemplaryembodiment of the invention. FIG. 2 is to scale. For simplicity thesupport structures holding the sources 52 and 54 as well as other partsof system 50 are not shown.

As shown in FIG. 2, the beams both illuminate the entire axial extent ofdetector 14, when they are energized. A controller 56 controls therotation of the sources and detector array 14 about an axis 58 andalternately energizes sources 52 and 54 and acquires data from thedetectors that form a part of detector array 14. Data from thedetectors, together with the rotation angle at which the data wasacquired and the source corresponding to the data is stored and used toreconstruct a CT image of a region 60.

Collimator 62 limits the extent of the beams so that the beams irradiatethe patient only within the solid angles covered by detector array 14.Collimator 62 may be fixed relative to the sources 52 and 54 anddetector 14 or it may be provided with means to move the collimatorleafs so as to change the geometry of the cone beams. In particular, inone mode of operation, one of the sources e.g. 52 is blocked and onesource e.g. 54 is used to irradiate subject 18 in the manner of theprior art CT of FIG. 1.

Further, in some embodiments sources 52 and 54 can be moved axially to adifferent position relative to detector 14. For example, if the sourcesare moved by half the source to source distance or source 52 is centeredabout the detector, source 54 is blocked and collimator 62 is adjustedappropriately, the scanner can be used in the prior art single symmetriccone beam mode.

Region 60 can be divided into three parts. In a first region 66 data isacquired from radiation by both sources 52 and 54. In regions 68 and 70data is acquired only from beams 52 or 54 respectively. In region 66,the image is reconstructed utilizing data collected from both beam, inas much as data is available from both sources for a particular volumeelement at a particular gantry angle. Though the setup may not fullymeet the Tuy-Smith condition, and data in region 60 may still beincomplete and somewhat truncated, still—given other resources andconditions substantially equivalent—better images with less cone beamartifacts may be generated as compared to a single source cone beamscanner as shown in FIG. 1 performing a circular trajectory of thesource.

In regions 68 and 70, the data from the respective single beam is usedfor reconstruction. In these regions the distance of any volume elementfrom the source trajectory planes are small and data incompleteness isminimal.

Axis 58 can be centered between the sources and detector to give amagnification of 2. Optionally, the center of rotation can be offsetfrom the center to give lesser or grater magnification. It is understoodthat while the magnification may be varied, the total extent of theimaged region depends on the distance between the sources, the geometryof collimator 62 and the coverage of detector array 14.

While in the embodiment shown in FIG. 2, some radiation is not utilizedfor reconstruction, the volume irradiated without producing image issubstantially smaller as compare to the prior art geometry shown in FIG.1.

In a non limiting exemplary embodiment the distance of sources 52 and 54from the rotation axis is 500 mm and the spacing between the sources is100 mm. Detector 14 includes 160 rows of detector elements with spacingof 1 mm between rows centers. The rows extend in the radial direction asparallel concentric arcs at a radius of 900 mm from the source to sourceaxis (the detector is shown as being flat in the axial direction, in allthe figures, for simplicity). The magnification of the system in thisexample is 1.8. For a scanned subject 18 with diameter of 250 mm, thecovered length 60 in one rotation is 125 mm. The outside angle 8 is then1.909 degrees.

The width of the overlap region 66, where at least for some rotationangles data is available from both sources, is 108.4 mm. The width ofregions 68 and 70 where data is available for all rotation angles fromone source only is 10.1 mm, for a total full coverage region of 118.5mm. The width of the beam at the exit is 128.6. The sources are rotatingat a frequency of four rotations a second about the subject. 1500 viewsare acquired from detector 14 for each rotation and the sources 52 and54 are energized alternatively under control of control unit 56 at afrequency of 3 KHz so each source generates 750 sets of data for eachrotation. For a partial rotation of about 200°, 418 sets of data areacquired for each source.

In some embodiments of the invention (as in the above example and asshown in FIG. 2), beam edges 64 may diverge away from the sources. Underthese circumstances some of the radiation may not be used inreconstruction. In the exemplary embodiment of the invention above about93% of the length irradiated along the scanner axis is used inreconstruction from a single rotation (region of fill coverage). Inother embodiments with different configurations, the ratio may varybetween 100% and 80%. In comparison, a prior art cone beam CT accordingto FIG. 1 with similar geometrical distances, subject 18 diameter of 250mm and entry width 22 of 125 mm, only about 75% of the irradiated lengthat the scanner center is used for reconstruction. In some embodimentsthe multiple cone beams may be symmetrical respective of the sourcetrajectory plane of each, wherein the inner parts of the cone beam areoverlapping for at least a part of the irradiated volume. It is notedthat as the length and diameter of the irradiated area increase, thepercent of length reconstructed increases for the various embodiments(if the sources are moved further apart) and decreases when a singlesource of the prior art is used. In addition, the size of the detectorarray of the prior art increases in size at a faster rate than those ofthe present invention.

FIG. 3 show a system 101 similar to system 50 in FIG. 2 wherein thedistance between sources is approximately the same as the axial extentof detector 14. System 101 maximizes the utility of radiation. It isnoted that the inner edges of the beams from the sources cross insidethe region of reconstruction. This does not cause serious reconstructionproblems, since views are taken from all angles so long as a 360 degreerotation is performed.

Furthermore, while in FIGS. 2 and 3 each of the beams is shown asilluminating the entire detector array, in some embodiments of theinvention, a portion of the array may be illuminated by only one of thebeams.

FIG. 4 shows a system 72 similar to that of FIG. 3 and on the samescale, in which the detector is extended so that a 180 degree plusrotation can provide a full data set. In system 72, the beam angle ofthe beams is increased. The detector size is increased to correspond tothe inner edge of the beam. This allows for full coverage of the regionof full coverage on a half rotation.

In other embodiments where the diameter of the reconstruction area islarger or small than that shown, the cone beam and area of the detectorthat is illuminated can be adjusted to change the amount of radiation tothe patient and/or amount of rotation needed for a complete data set.Furthermore, where the magnification is changed by changing the positionof the reconstructed portion and center of reconstruction, theillumination of the detectors may vary between a situation in which theouter portions of the detectors are illuminated by only the beamopposite (as for example where the magnification is low and the centerof rotation is moved toward the detectors) and the situation shown inFIG. 4. In some cases the detector is illuminated over its entire lengthby both beams.

It is understood that the collimators can be used to shape the innerangles of the beams to provide an optimum angle, in some embodiments ofthe invention.

In exemplary embodiments of the invention, a variation of the cone beamangle and offset will effect the reconstructed volume in patient 18.

The detector elements that make up detector array 14 can comprise anydetection elements as known in the art, for example, comprising CdW₄crystals coupled to Silicon photodiodes detection, or any otherradiation detection scheme suitable for CT as known in the art.

FIG. 5 schematically illustrates a CT scanner 100 similar to 50 of FIG.2, comprising three cone beam sources 82, 84 and 86. The offsets betweenthe sources are not necessarily equal.

FIG. 6 schematically illustrates a cross-section trans axial view of aCT scanner 201 comprising a group of multiple cone beam sources 203, 205offset from each other circumferentially about rotation axis 58, eachwith a corresponding detector array 14 and 14′, respectively, inaccordance with exemplary embodiments of the invention. It should beunderstood that preferably each of multiple cone beam sources comprisestwo or more axially offset sources as shown in FIGS. 2-5. It should beunderstood that larger number of multiple cone beam sources can beprovided, e.g. 3 groups or more and the groups can disposed about therotation angle at equal angular spacing or at other angular positions.

Further, in FIG. 6 the detector arrays are shown to be rotating alongwith the sources but in some embodiments a fixed array of detectors isprovided where the sources are rotated. To reduce effects of scatteredradiation originated from one multiple cone beam source and impinging onthe detector corresponding to another multiple cone beam source,anti-scatter grids are provided in front of the detectors as known inthe art. In addition, sources 203 and 205 may be energizedalternatively, in addition to the switching of radiation between conebeams within each multiple source.

In some embodiments described by FIG. 6 the multiple sources 203 and 205are mounted at a same axial position relative to the scanned subject.Thus, for example if source 203 emits two alternating axially spacedcone beams and source 205 emits two alternating axially spaced conebeams, each focal spot in source 203 moves in rotation on a samecircular trajectory as a corresponding focal spot on source 205. Each ofthe corresponding pairs of focal spots moves on a parallel trajectorywith the other pair of corresponding focal spots. In one mode ofoperation data collected by detector 14 while the sources are rotatedover rotation angle smaller than needed for image reconstruction isadded to data collected by detector 14′ alone during the same time so asto form a data set useful for reconstruction. In this mode, temporalresolution is improved relative to system with one multiple source as inFIG. 2-5. In another mode of operation, data collected from detectors 14and 14′ is added or averaged to improve statistics and reduce imagenoise.

In some other embodiments included in FIG. 6 multiple sources 203 and205 are displaced axially as well as circumferentially. Thus, forexample if the axial displacement is equal to half the spacing betweenthe focal spots, the focal spots of the two sources 203 and emits twoalternating cone beams and source 205 emits two alternating cone beams,the four focal spots move on circular trajectories that are spaced byone-half the source spacing in each of sources 203 and 205, with thecone beams of the two sources interleaved. Parts of the subject 18 areirradiated by radiation from two or more focal spots and thecorresponding image is reconstructed from weighted data acquired frommultiple focal spots. This results in increased resolution.

Alternatively, if the centers of 203 and 204 are axially spaced by asame distance as the distance between the focal spots in each of 203 and205, then an axial extent double that of a single unit can be imaged inone half rotation.

Alternatively, the axial spacing can be equal to twice the sourcedistance. In this case the focal spots of the cone beams rotate onparallel trajectories that are one source distance apart. If, instead ofthe asymmetric cone beams shown above, the cone beams are symmetric,then the region between the sources 203 and 205 is irradiate by one conebeam from source 203 and one from source, 205 in much the same way as isthe region between the focal spots on a same source 203 or 205. Thisresults in a larger swath being scanned in each rotation.

Thus depending on the amount of axial displacement for the two sets ofsources, faster data acquisition, higher resolution or a larger swathare possible.

FIG. 7 schematically illustrates a cross-section trans axial view of anapparatus for CT scanner 201′ comprising a group of multiple cone beamsources 203′ and 205′ offset from each other in a directionperpendicular to rotation axis 58, and sharing a common curved detector14, in accordance with exemplary embodiments of the invention. It shouldbe understood that preferably each of multiple cone beam sourcescomprises two or more axially offset sources as shown in FIG. 2, 3 or 4.

In some embodiments of the systems described in FIG. 2-7 each of thefocal spots generating the cone beams is moved periodically duringradiation, either between discrete positions or by periodic motion anddata acquisition is synchronized to collect data separately fordifferent focal spots position. Focal spot deflection can beaccomplished by electrostatic or magnetic deflection of electron beamimpinging on the anode surface. The use of focal spot deflection forimprovement of image resolution is well known in the art as describede.g. in U.S. Pat. No. 4,637,040, the disclosure of which is incorporatedinhere by reference. The focal spot deflection can be in the axialdirection or in the trans-axial direction or both, providing improvedresolution in the corresponding direction.

In exemplary embodiments of the invention, a CT scanner may comprise aplurality of configurations of the kind illustrated in FIGS. 2-7.Optionally, the scanner may comprise a combination of the configurationsshown in FIGS. 2-7 or their variants. In exemplary embodiments of theinvention, detector 14 comprises a cylindrical ring of fixed detectorswhile the sources rotate about rotation axis 58. Alternatively, both thesources and detectors are fixed and the object being imaged rotates.This construction is especially useful for industrial imaging.

Optionally, the sources and detector array 14 are connected rigidly.Optionally or alternatively, at least a portion of the movement of thesources and the detector array is synchronized substantially without amechanical connection.

In exemplary embodiments of the invention, the sources are switched onand off sequentially, such that radiation is emitted alternatively bythe sources. Alternatively, the sources are energized immediately aftereach other and then energized in a similar way as the sources pass fixedincrement in their rotation about axis 58.

In exemplary embodiments of the invention, a sequence of X-ray emissionsis generated such that all of the data is acquired when the heart isrelatively quiescent, e.g. at a particular phase relative to the Rsignal of an ECG, by synchronization to an ECG or blood pressure signal.In exemplary embodiment of the invention the data is acquired as a heartcycle phase centered about 70% of the R-R interval or about anotherphase in the range of 65% to 75% of the R-R interval.

In exemplary embodiments of the invention, the sources (e.g., 52 and 54)radiate cyclically so that an alternation cycle of the plurality ofsources is short relative to the rotation or scanning time. For example,in the order of kilocycles per second (kHz) as described below in moredetail.

The relative rotational motion about axis 58 while irradiating subject18 and acquiring data may comprise a full circle or a half circle or ahalf circle plus additional angular range or any other rotation angle asrequired for reconstruction of the object, as known in the art.

Further, in some embodiments of the invention the sources 52 and 54 areenergized and data acquired for time extent comparable or exceeding awhole heart period. In exemplary embodiment the rotation speed isseveral rotations per heart beat and acquisition lasts for longer than aheart beat period. Images are reconstructed for data sets correspondingeach to rotation angle as needed for image reconstruction with angularand temporal increment between the sets. Thus a sequence of images isgenerated, corresponding to successive phases in the heart cycle.

Further, in some embodiments of the invention the sources 52 and 54 areenergized and data is acquired multiple times, during multiple heartperiods wherein each single acquisition is over a rotation angle whichis optionally smaller than needed for image reconstruction. Data frommultiple heart beats acquired at a particular phase of the heart periodare added together to form a data set for image reconstruction. Theseembodiments provide image acquired over a shorter part of the cardiaccycle thereby improving temporal resolution.

Further, in some embodiments of the invention the sources 52 and 54 areenergized and data is acquired multiple times for a period of 180+degrees, during multiple heart periods. Data from multiple heart beatsacquired at a particular phase of the heart period are weightedaccording to redundancy and used together for image reconstruction.Alternatively, each data set is reconstructed to image and images areaveraged. These embodiments provide images with reduced noise asrequired e.g. for large subjects wherein the radiation from sources 52and 54 is highly attenuated and the data received by detector 14 has ahigh statistical error.

Optionally, the region of interest is completely covered by radiationfrom the sources and may be reconstructed by circular motion at oneposition relative to rotation axis 58. Optionally, data is acquired atone axial position, subject 18 is moved along rotation axis 58 and datais acquired at a second axial position, the sequence optionally repeatedas required to cover the region of interest. Optionally said motion ofsubject is by less than the width covered by radiation in one positionso parts of subject 18 are irradiated both at one subject position andat a second subject positions. The image of such overlap region may bereconstructed from weighted data acquired from different sources.Optionally, for this embodiment, the beams are symmetric and identicaland the movement is twice the spacing between the sources. Then the formof the radiation between the second source in the first position and thefirst source in the second position is the same as the radiation betweentwo sources at a same position.

In a non limiting exemplary embodiments of the invention useful forscanning the human heart, the source's focal points are separated by 120mm (range 60 to 140 mm) at a distance 500 mm from axis 58 (SAD) (range400 to 700 mm). Detector 14 comprises a circular arc with a radius of900 mm (range 700-1000 mm) from the line connecting the focal points ofthe sources (SDD), so that all beam lengths are approximately the same.With this geometry, the system has a magnification of 900/500=1.8 (range1.5 to 2.2). In an embodiment of the invention, detector 14 has an axialextent of 162 mm (range 100-250 mm) comprising 180 rows of 0.9 mmelements (range 0.5 to 1.5 mm, number of rows derived from extent andelement size). Alternatively, smaller size elements may be provided,e.g. size 0.2 mm or 0.3 mm and in certain modes of operations elementare binned together to generate summed output signal. The extent aboutthe isocenter (rotation axis 58) is 120+(162−120)/1.8=143 mm. Thisextent may cover a whole heart or its substantial part in one scanningrotation.

For comparison, a similar geometry and magnification with a singlesource covering the same 143 mm extent about the rotation axis wouldrequire a detector with an extent of 143×1.8=257 mm and 286 rows,typically more demanding mechanically, more expensive and thereconstruction is more problematic as the cone angle is wider and moredata is off the central beam, thus less conforming with the Tuy-Smithcondition.

As for temporal resolution, assuming a heart rate at about 60 beats persecond, or about 1000 ms per beat, a scan rotation of about 250 ms withat least 500 data samplings per rotation (for each source) may wellcapture the heart substantially as a snapshot (frozen), especially ifthe scan is synchronized with ECG or other signal to capture the heartjust during a period of low movement.

Consequently, assuming a duty cycle of 50% for each source, each on oroff time is 250/(500×2)=0.25 ms, and a switching rate of 2 KHz isrequired of each source.

Taking into account the two simultaneous scanning trajectories, apartial circular scan may be sufficient. Assuming a rotation about 220°,then the required scan time will be only 250×220/360=153 ms, enhancingthe temporal resolution.

This radiation switching rate may be achieved, for example, by a tubewith two or more anodes and one or more cathodes, where the electronsflow from a cathode to an anode is alternated by an alternating negativepotential applied to one or more grid electrodes. Another example is twoconventional tubes with alternating high voltage supply which energizesthe two tubes alternatively. Another example is by steering theelectrons over one or more anodes.

Yet another example of apparatus for producing alternating radiationfrom two sources is to block and unblock the radiation paths of thebeams mechanically and/or pneumatically and/or electro-mechanically, forexample by perforated rotating wheel comprising of radiation blockingmaterial such as lead or tungsten.

An exemplary X-Ray tube for use in the present invention is describedbelow. In the disclosed X-ray tube, the two sources are in the samevacuum enclosure and electron beams are gated by grid electrodes to oneor the other source focal point.

The exemplary embodiments described above are examples of arrangementand operation of embodiments of the present invention, but are notlimiting in any respect whatsoever and any parameter may be changed toachieve some performance, size or cost criterions.

FIG. 8 schematically illustrates an anti-scatter grid 400 with septa 402aligned substantially parallel to rotation axis 58 and radially tosources 52 and 54 axis.

Septa 402 allow direct radiation 404A from source 52 or 404B from source54 to impinge on detector elements in detector column 408. Beamsscattered by patient 18, such as beam 406 are blocked from impingingdetector elements 408.

Optionally, grid 400 comprises a part of detector 14. Optionally oralternatively, grid 400 is attachable to, and optionally detachablefrom, detector 14.

FIG. 9A schematically depicts the relevant portions of an exemplaryX-ray tube 200 according to an embodiment of the invention.

In the depicted embodiment, a single vacuum enclosure 210 houses twoanodes 220 with a cathode assembly 230 positioned between them. FIG. 9Bshows a cross-sectional view of the cathode assembly structure.

Optionally, anodes 220 are rotating anodes. In an exemplary embodimentof the invention, anodes 220 are mounted on a common rotation axis 250within vacuum enclosure 210. For simplicity, the means for rotating theanodes are not shown. However, any method for rotating anodes in X-Raytubes, as known in the art, can be used. Similarly, the electricalconnections of the anode(s) and cathode(s) are not shown for simplicity.Making such connections is well known in the art. Furthermore, anysuitable construction of the anodes as known in the art can be used.

On one embodiment of the invention, cathode assembly 230 includes asingle cathode filament 232. In one embodiment of the invention, thecathode is grounded, and the anode is at a high positive voltage. Inothers the anodes are grounded and the cathode is at a high negativevoltage. Preferably, both the cathode and anode are electrified withnegative and positive voltages respectively. This is preferred becauseit is easier to handle two voltages of (for example) 70 kW then onevoltage of 140 kW. In any event, the difference in voltage between thecathode and anode depends on the application in which it is being used.For a medical CT application, the voltage is in the range of 80 kV and140 kV. Heating of filament 232 causes cathode 230 to thermally emitelectrons which form a cloud around the emitting surface of the cathode.The electrons, accelerated and impinging on targets 260 and 262 giverise to emission of X rays. Vacuum 210 is provided with ports/windowsrelatively transparent to X radiation that allow X ray beams to beemitted out of the enclosure. The shape and size of the ports determinesthe angular extent of the beams. Thus, the window opening (and externalcollimators) define the type of beam. For the present invention, theopening defines a cone beam with overlapping beams as shown in FIGS.2-4.

In the depicted embodiment, voltage gates 240 are provides as part ofcathode assembly 230 and are capable of modulating a beam of electronsthat impinge targets 260 and 262 on anodes 220. The depictedconfiguration insures that a beam of thermally emitted electrons fromfilament 232 must pass the electrostatic field generated by gates 240 toarrive at targets 260 and 262. In an exemplary embodiment of theinvention, each voltage gate 240 comprises a pair of electrodes (e.g.242 and 244 or 246 and 248, as shown in FIG. 9B). Each pair ofelectrodes is electrically isolated from the filament. In someembodiments the electrodes within each pair are electrically connected.In other embodiments they are insulated from each other so there can beapplied a bias voltage between them.

The gates and/or the cathodes may include additional electrodes toconfine and/or to form the beam, as is well known in the art. Theseadditional electrodes are depicted very schematically as a pair ofplates 290 for each target. As indicated below electrodes 290 can beused for focusing, gating and other functions. For simplicity ofpresentation, electrodes 290 are shown only in FIG. 9A and are omittedfrom FIGS. 9B-E. However, such electrodes can be applied to any of theembodiments shown.

Two beams, very schematically shown at 292 in FIG. 9B are generallyalternately generated as described below. The support structure of theelements is not shown, for simplicity of presentation. Optionally, onlythose portions of cathode 230 facing targets 260 and 262 are emitting.In an exemplary embodiment of the invention, the gates are connected tocontrol circuitry 310; described below with reference to FIGS. 10A, 10B,11A and 11B.

FIG. 9C shows an X-Ray tube 200′ having an alternative cathodeconstruction, in accordance with an embodiment of the invention. In thisembodiment cathode assembly 230′ has separate emitters 232′ and 232″(each with its own filament) used to produce the electron beams thatpass their respective gates 240. Alternatively, the cathodes can share acommon filament.

This construction facilitates the placement of electrodes (not shown)for producing a more focused beam to impinge on target areas 260 and 262and also makes it possible to position the filaments 232′ and 232″parallel to X ray emitting surfaces of anodes 260 and 262, respectively(not shown). FIG. 9D is a cross-sectional view of the cathode assemblyof FIG. 9C.

FIG. 9E shows an alternative construction of the anode portion of anX-Ray tube 200″ on a partly cross-sectional view, In accordance with anembodiment of the invention. In this embodiment, a single anodestructure 220′ rotated around a shaft 250′ contains both targets 260 and262. This construction is simpler. However, since both beams impinge onthe same anode structure, the total heat capacity (and the maximum poweroutput) is reduced considerably. The cathode assembly of either FIG.9A/B or 9C/D can be used with this embodiment. Cathode assembly 230 isshown as a box, to indicate that the constructions of either FIG. 9A/Bor FIG. 9C/D can be used.

Although rotating anodes are generally needed for CT, as indicatedabove, the present invention is also usable for other applications, inwhich case stationary anode structures can be used.

FIG. 10A shows a simplified block diagram of an exemplary system 202 forenergizing the X-Ray tubes of FIGS. 9A-E where both elements of eachgate are at the same potential. The block diagram shown in FIG. 10Arepresents just one way to energize the tube. In general, anymethodology which allows for the application of a high voltage betweenthe anode and cathode and which allows for switching the voltagesbetween cathode and gate between blocking and transmitting voltages foran electron beam can be used.

System 202 comprises a high voltage part 102, a filament current part103, a gate voltage part 104 and a control 112.

In most systems high voltage part 102 comprises a high voltage powersupply connected between the cathode(s) and anode(s). As indicated abovethe anode or cathode (or neither) can be grounded. For this reason nogrounding is shown in FIG. 10A

Filament current part 103 is a current power supply generating thecurrent needed to heat the filament in order to induce thermal electronemission.

In the embodiment shown gate voltage part comprises a gate modulatorthat generates two square wave modulated output voltages, each connectedto one of gates 240. In a preferred embodiment the gate modulator outputvoltages are controllable in the range 0 to −5000 V respective of thefilament and modulate at a high frequency in a square wave form betweena low and high levels selectable for each gate (see FIG. 11A). The gatevoltage levels are selected such that the highly negative level issufficient to block the flow of electrons from cathode to anode and cutoff the X rays. The less negative level may be 0 or a value sufficientto focus the electron beam to a desired size but yet to transmit theelectron beam. A gate modulator has two states. In a first state the cutoff voltage is applied to a first gate 240′ and the transmit voltage isapplied to a second gate 240. In a second state, the cut off voltage isapplied to second gate 240 and the transmit voltage is applied to firstgate 240. Thus, in the first state an electron beam impinges a secondtarget 262 (associated with gate second gate 240) and there is no beam afirst target 260. In the second state an electron beam impinges thefirst target and not the second target. This results in a controllableexcitation of the first and second targets to produce x-rays inalternation.

FIG. 10B is a block diagram of an exemplary circuit 202′ forelectrifying a tube in which the gate electrodes for each gate areseparate and isolated from each other. Cut-off is determined by stronglybiasing both electrodes of a set. When the electrodes are differentiallyelectrified, and the beam is not cut off, the difference results insteering of the beam at an angle toward the electrode with the morepositive voltage. This results in the beam impinging on the anode at atarget point that is slightly displaced along the circumference of theanode. The result is the possibility of generating sets of x-ray sourceseach set of which is generated by the electrodes in a particular gateand displaced in a first direction by a displacement typically of 5-15cm. Within the set, the positions of the target/sources can be varied ina direction perpendicular to the first direction by applyingdifferential voltages to the two elements of the gate. The center offocal spot position may typically be steered by 1 mm or more or less.

As shown in FIGS. 9B and 9D, electrodes 242, 244, 246 and 248 have atriple function, namely to gate the beam on and off, to focus the beamand to steer the beam. As is known in the art, these functions can bedivided among different electrodes 290 and grids (not shown), and thepresent invention includes such variations. As shown the circuitry shownin FIG. 10B is capable of generating independent voltages for each ofthe four electrodes.

In an embodiment of the invention control 112 control grid modulator 106and optionally HV PS 102. Control may also have an output whichtransmits the current state of circuit 202′ to a data acquisition systemor a computer which receives signals corresponding to attenuation of thex-rays generated by tube 200.

In a fast CT scanner rotation time of the gantry may be of the order of250 to 500 ms. Generally, to acquire a full set of views using a conebeam, rotation of 180°+ twice the cone angle is required. In order toacquire 1000 views per rotation for each of two focal spots, therequired switching frequency is 2 to 4 kHz. For generating multiplebeams in each set, a higher modulation rate would be required. Inpractice, depending on the rotation rate and other factors, themodulation frequency could be between 1-4 kHz, or even between 500 Hzand 8 kHz

For use in angiography applications the frequency may be 25/30 or 50/60Hz or a higher or lower value.

Optionally, a low negative transmit voltage, such as −1.0 or −2.0 kV isapplied to open gates 240 depending on the geometry of the cathode,anode and gate. In an exemplary embodiment of the invention, applicationof a low negative voltage to an open gate 240 focuses the electron beamand/or controls a focal spot dimension. Optionally the cut-off voltagefor a tube operating at 120 kV is in the range −4000 or −5000 volts.

FIGS. 11A and 11B shows switching voltages for the circuits of FIGS. 10Aand 10B respectively. In FIG. 11B, voltages are provided for two sourcepositions of x-ray on each anode, with the focal spot being modulatedbetween the four positions each cycle. Beam deflection is not limited todiscrete focal spots (square wave). Sine wave deflection or other ispossible as well.

Note that while the electrodes shown can be used to deflect the beam inonly one direction. It is possible to deflect the beams in the seconddirection (up-down in FIGS. 9A, 9C and 9E) by means known in the art,such as for example, a pair of deflecting plates situated between thecathode and anode. Alternatively, the beam may be deflected using avarying magnetic field. Alternatively or additionally, the primarydeflection, described above using two electrodes, can be performed usingeither plate electrodes or magnetic fields.

In an exemplary embodiment of the invention, where the tube is used in acone beam CT scanner, a distance between the focal spots and the axis ofrotation (SAD) is 500 mm. This optional configuration provides coverageof 140 mm at isocenter. In general in wide angle cone beam CT a coneopening in the Z direction (for each x-ray beam) is +6-15°. In anexemplary embodiment of the invention, the anode angle should be atleast 8-18° to avoid a heel effect under these conditions. In some usesof the present invention (such as, for example that shown above, wherethe beam is asymmetric (with a greater angle toward the cathode), theanode angle can be considerably smaller. In other embodiments, bothbeams are symmetrical. In the embodiment of FIG. 5, the beam of thecentral source is symmetrical. In an exemplary embodiment of theinvention, a short exposure time is used for imaging of a rapidly movingtissue (e.g. cardiac tissue). Optionally, an increase in a power ratingof X-ray tube 200 contributes to an ability to achieve an acceptableimage contrast with a short exposure time. In an exemplary embodiment ofthe invention, use of alternating anodes 220 as described above in FIGS.9A and 9C, contributes to a reduction in anode heating. Optionally, thereduction in anode heating contributes to a feasibility of using a tubewith a higher power rating. In the embodiment depicted in FIGS. 9A and9C, heat load on each of anodes 220 are 50% of comparable loads in aconventional single anode X-ray tube.

One of ordinary skill in the art will be able to select and configureother portions of X ray tube 200 and/or control circuitry 56 such as ahousing and/or regulators and/or splitter and/or power supply and/orcabling and/or high voltage connectors from commercially availablecomponents. For clarity, these portions of X-ray tube 200 and/or controlcircuitry 56 are not shown and/or represented only schematically.

A variety of numerical indicators have been utilized to describe variouscomponents of the X-ray tube and/or voltages and/or power inputs. Itshould be understood that these numerical indicators could vary evenfurther based upon a variety of engineering principles, materials,intended use and designs incorporated into the invention. Additionally,components and/or actions ascribed to exemplary embodiments of theinvention and depicted as a single unit may be divided into subunits.Conversely, components and/or actions ascribed to exemplary embodimentsof the invention and depicted as sub-units may be combined into a singleunit with the described/depicted function.

It should be further understood that the individual features describedhereinabove can be combined in all possible combinations andsub-combinations to produce exemplary embodiments of the invention. Theexamples given above are exemplary in nature and are not intended tolimit the scope of the invention which is defined solely by thefollowing claims. Specifically, the invention has been described in thecontext of a left atrium but might also be used in a right atrium or aventricle.

As used in the description and claims of the present invention, the term“cone beam” means a beam having a cone angle (in the direction of theaxis of the scanner) or at least two degrees.

In the description and claims of the present application, each of theverbs “comprise”, “include” and “have” as well as any conjugatesthereof, are used to indicate that the object or objects of the verb arenot necessarily a complete listing of members, components, elements orparts of the subject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to necessarily limit the scope of the invention. In particular,numerical values may be higher or lower than ranges of numbers set forthabove and still be within the scope of the invention. The describedembodiments comprise different features, not all of which are requiredin all embodiments of the invention. Some embodiments of the inventionutilize only some of the features or possible combinations of thefeatures. Alternatively and additionally, portions of the inventiondescribed/depicted as a single unit may reside in two or more separatephysical entities which act in concert to perform the described/depictedfunction. Alternatively and additionally, portions of the inventiondescribed/depicted as two or more separate physical entities may beintegrated into a single physical entity to perform thedescribed/depicted function. Variations of embodiments of the presentinvention that are described and embodiments of the present inventioncomprising different combinations of features noted in the describedembodiments can be combined in all possible combinations including, butnot limited to use of features described in the context of oneembodiment in the context of any other embodiment. The scope of theinvention is limited only by the following claims.

All publications and/or patents and/or product descriptions cited inthis document are fully incorporated herein by reference to the sameextent as if each had been individually incorporated herein byreference.

1. A method for CT cone beam scanning, comprising: (a) relativelyrotating a plurality of alternating cone beam X-Ray radiation sources toan object about a rotation axis, the sources being spaced parallel tothe rotation axis, and radiating through a common portion of the objectonto a common detector array having axially spaced rows of detectorelements, wherein the number of rows is greater than the number ofplurality of sources, by at least a factor of two; (b) acquiring from atleast a part of the detector, data responsive to radiation attenuated byat least part of a common volume of the object irradiated by theplurality of radiation sources; and (c) collimating the cone beams fromtwo sources of said plurality of sources such that at least 80% of thelength irradiated along the rotation axis is used for reconstruction ofthe object, wherein all of the rows are illuminated by at least twosources.
 2. A method according to claim 1 and comprising: (d)reconstructing an image of the object where the image of at least partof the common volume of the object is reconstructed from attenuationdata originating from said plurality of radiation sources.
 3. A methodaccording to claim 1 wherein said data is acquired in a single rotationor less of said sources relative to object.
 4. A method according toclaim 1 wherein acquiring data is limited to a period of rotation of 220degrees or less.
 5. A method according to claim 1 wherein all of therows are illuminated by each of the sources.
 6. A method according toclaim 1 wherein one edge of at least one asymmetrical cone beam isperpendicular to the detector array.
 7. A method according to claim 1,and including alternately energizing the radiation sources.
 8. A methodaccording to claim 7, wherein the plurality of sources radiate in cyclesso that an alternation cycle of the plurality of sources is shortrelative to a time of CT rotation or scanning.
 9. A method according toclaim 1, wherein the data is acquired in synchronization with at leastone of an ECG signal or blood pressure signal.
 10. A method according toclaim 1, wherein the x-ray sources revolve around the object a pluralityof times and data from different revolutions is averaged to reduce noisein a reconstructed image.
 11. A method according to claim 1 wherein thex-ray sources revolve around the object a plurality of times and whereindata from different revolutions is binned to form data from at least oneportion of a heart cycle and wherein said binned data is used toreconstruct a heart in at least one phase of the heart cycle. 12.Apparatus for CT cone beam scanning, comprising: a table for holding asubject; a gantry; a first detector array, having a given number of rowsof detector elements spaced along a rotation axis of the gantry, mountedon the gantry; a first plurality of cone beam x-ray sources mounted onthe gantry for rotation about the table on the rotation axis, the numberof rows being at least twice the number of sources; a controller thatacquires data responsive to X-Ray radiation from the sources from thedetector array attenuated by at least part of a common volume of thesubject irradiated by the plurality of radiation sources; and acollimator that collimates the cone beams from two sources such thatthey each are asymmetrical respective of a plane defined by rotation ofeach respective source about said axis, and such that at least 80% ofthe length irradiated along the rotation axis is used for reconstructionof the subject, wherein, all of the rows are illuminated by at least twosources.
 13. Apparatus according to claim 12 wherein the controllerreconstructs an image of the subject where the image of at least part ofthe common volume of the subject is reconstructed from attenuation dataoriginating from the plurality of radiation sources.
 14. Apparatusaccording to claim 13 wherein the controller is adapted to reconstructsaid data acquired in a single rotation or less of said sources relativeto subject.
 15. Apparatus according to claim 13 wherein substantiallyall of the radiation illuminating the subject is used in thereconstruction.
 16. Apparatus according to claim 12 wherein acquiringdata is limited to a period of rotation of 220 degrees or less. 17.Apparatus according to claim 12 wherein all of the rows are illuminatedby each of the sources.
 18. Apparatus according to claim 12 wherein oneedge of at least one of the asymmetrical cone beams is perpendicular tothe detector array.
 19. Apparatus according to claim 12, wherein thecontroller includes a power supply alternately energizing the X-Rayradiation sources and the plurality of sources radiate in cycles so thatan alternation cycle of the plurality of sources is short relative to atime of CT rotation or scanning.
 20. Apparatus according to claim 12 andfurther comprising: a second detector array, having a given number ofrows of detector elements spaced along an rotation axis of the gantry,mounted on the gantry; a second plurality of x-ray sources mounted onthe gantry for rotation about the table on the rotation axis, the numberof rows being at least twice the number of sources; and a controllerthat acquires data responsive to radiation from the first and secondplurality of sources from both the first and second detector arraysattenuated by at least part of the common volume of the subjectirradiated by the first and second plurality of radiation sources. 21.Apparatus according to claim 20 wherein the second detector array andthe second plurality of x-ray sources are circumferentially offset fromthe first detector array and first plurality of x-ray sources. 22.Apparatus according to claim 21 wherein the second detector array andthe second plurality of x-ray sources are not axially offset from thefirst detector array and first plurality of x-ray sources.
 23. Apparatusfor CT cone beam scanning, comprising: a table for holding a subject; agantry; a first detector array, mounted on the gantry and having a givennumber of rows of detector elements spaced along a rotation axis of thegantry; a first plurality of x-ray cone beam sources mounted on thegantry for rotation about the table on a rotation axis, the number ofrows being at least twice the number of sources; a controller thatacquires data responsive to radiation from the sources from the detectorarray attenuated by at least part of a common volume of the subjectirradiated by the plurality of x-ray sources; and a collimator thatcollimates the cone beams such that the cone beams from two of saidplurality of sources are asymmetrical with respect to a plane defined byeach respective source trajectory about the rotation axis and the edgeof at least one asymmetrical cone beam which is farther from the othersource makes a larger angle to the detector array than does the beamedge closer to the other source; and such that the farther edge of theat least one cone beam makes an angle of between 87 and 90 degrees withthe detector array.
 24. Apparatus according to claim 23 wherein thecollimator collimates the beams such that at least 80% of the lengthirradiated at said rotation axis by said at least one asymmetrical conebeam is, used in reconstruction of subject.
 25. Apparatus according toclaim 24 wherein at least 90% of the length of the rotation axis is soused.
 26. Apparatus according to claim 25 wherein the collimatorcollimates the beams such that the farther edge of the at least one beamsubstantially coincides with said planes such that 100% of the area ofthe at least one beam is so contained.
 27. Apparatus according to claim23 wherein the collimator collimates the beams such that the anglebetween the edge of the beam closer to the other source with thedetector array is between 75 and 84 degrees.
 28. Apparatus according toclaim 23 wherein the collimator collimates the beams such that the outeredge of each of the two beams substantially coincides with said planessuch that 100% of the area of the beams is so contained.
 29. Apparatusaccording to claim 23 wherein the collimator collimates the beams suchthat the angle between the distal beam edges of the two beams is between0 and 4 degrees.